Cloning and characterization of a novel periplasmic heme-transport protein from the human pathogen Pseudomonas aeruginosa

The role of host heme in bacterial infection

Heme is an indispensable cofactor for almost all aerobic life, including the human host and many bacterial pathogens. During infection, heme and hemoproteins are the largest source of bioavailable iron, and pathogens have evolved various heme acquisition pathways to satisfy their need for iron and heme. Many of these pathways are regulated transcriptionally by intracellular iron levels, however, host heme availability and intracellular heme levels have also been found to regulate heme uptake in some species. Knowledge of these pathways has helped to uncover not only how these bacteria incorporate host heme into their metabolism but also provided insight into the importance of host heme as a nutrient source during infection. Within this review is covered multiple aspects of the role of heme at the host pathogen interface, including the various routes of heme biosynthesis, how heme is sequestered by the host, and how heme is scavenged by bacterial pathogens. Also discussed is how heme and hemoproteins alter the behavior of the host immune system and bacterial pathogens. Finally, some unanswered questions about the regulation of heme uptake and how host heme is integrated into bacterial metabolism are highlighted.

Impact of Na+-Translocating NADH:Quinone Oxidoreductase on Iron Uptake and nqrM Expression in Vibrio cholerae

The Na⁺ ion-translocating NADH:quinone oxidoreductase (NQR) from Vibrio cholerae is a membrane-bound respiratory enzyme which harbors flavins and Fe-S clusters as redox centers. The NQR is the main producer of the sodium motive force (SMF) and drives energy-dissipating processes such as flagellar rotation, substrate uptake, ATP synthesis, and cation-proton antiport. The NQR requires for its maturation, in addition to the six structural genes nqrABCDEF, a flavin attachment gene, apbE, and the nqrM gene, presumably encoding a Fe delivery protein. We here describe growth studies and quantitative real-time PCR for the V. cholerae O395N1 wild-type (wt) strain and its mutant Δnqr and ΔubiC strains, impaired in respiration. In a comparative proteome analysis, FeoB, the membrane subunit of the uptake system for Fe²⁺ (Feo), was increased in V. cholerae Δnqr In this study, the upregulation was confirmed on the mRNA level and resulted in improved growth rates of V. cholerae Δnqr with Fe²⁺ as an iron source. We studied the expression of feoB on other respiratory enzyme deletion mutants such as the ΔubiC mutant to determine whether iron transport is specific to the absence of NQR resulting from impaired respiration. We show that the nqr operon comprises, in addition to the structural nqrABCDEF genes, the downstream apbE and nqrM genes on the same operon and demonstrate induction of the nqr operon by iron in V. cholerae wt. In contrast, expression of the nqrM gene in V. cholerae Δnqr is repressed by iron. The lack of functional NQR has a strong impact on iron homeostasis in V. cholerae and demonstrates that central respiratory metabolism is interwoven with iron uptake and regulation.IMPORTANCE Investigating strategies of iron acquisition, storage, and delivery in Vibrio cholerae is a prerequisite to understand how this pathogen thrives in hostile, iron-limited environments such as the human host. In addition to highlighting the maturation of the respiratory complex NQR, this study points out the influence of NQR on iron metabolism, thereby making it a potential drug target for antibiotics.

Identification of the heme acquisition system in Vibrio vulnificus M2799

Vibrio vulnificus, the causative agent of serious, often fatal, infections in humans, requires iron for its pathogenesis. As such, it obtains iron via both vulnibactin and heme-mediated iron-uptake systems. In this study, we identified the heme acquisition system in V. vulnificus M2799. The nucleotide sequences of the genes encoding heme receptors HupA and HvtA and the ATP-binding cassette (ABC) transport system proteins HupB, HupC, and HupD were determined, and then used in the construction of deletion mutants developed from a Δics strain, which could not synthesize vulnibactin. Growth experiments using these mutants indicated that HupA and HvtA are major and minor heme receptors, respectively. The expressions of two proteins were analyzed by the quantitative reverse-transcriptase polymerase chain reaction (qRT-PCR). Furthermore, complementation analyses confirmed that the HupBCD proteins are the only ABC transport system shared by both the HupA and HvtA receptors. This is the first genetic evidence that the HupBCD proteins are essential for heme acquisition by V. vulnificus. Further investigation showed that hupA, hvtA, and hupBCD are regulated by Fur. The qRT-PCR analysis of the heme receptor genes revealed that HupR, a LysR-family positive transcriptional activator, upregulates the expression of hupA, but not hvtA. In addition, ptrB was co-transcribed with hvtA, and PtrB had no influence on growth in low-iron CM9 medium supplemented with hemin, hemoglobin, or cytochrome C.

Thoughts on interactions between PGRMC1 and diverse attested and potential hydrophobic ligands

Progesterone Receptor Membrane Component 1 (PGRMC1) is located in many different subcellular locations with many different attested and probably location-specific functions. PGRMC1 was recently identified in the mitochondrial outer membrane where it interacts with ferrochelatase, the last enzyme in the heme synthetic pathway. It has been proposed that PGRMC1 may act as a chaperone to shuttle newly synthesized heme from the mitochondrion to cytochrome P450 (cyP450) enzymes. Here we consider potential roles that PGRMC1 may play in transferring heme, and other small hydrophobic ligands such as cholesterol and steroids, between the hydrophobic compartment of the membrane lipid bilayer interior to aqueous proteins, and perhaps to the membranes of other organelles. We review the synthesis and roles of especially PGRMC1- and cyP450-bound heme, the sources and transport of cholesterol, the involvement of PGRMC1 in cholesterol regulation, and the production of the first progestogen pregnenolone from cholesterol. We also show by clustering by inferred models of evolution (CLIME) analysis that PGRMC1 and related proteins exhibit co-evolution with a series of cyP450 enzymes, as well as a group of mitochondrial proteins lacking in several parasitic protist groups. Altogether, PGRMC1 is implicated with important roles in sterol synthesis and energy regulation that are dispensable in certain parasites. Some novel hypothetical models for PGRMC1 function are proposed to direct future investigative research.

HmuS from Yersinia pseudotuberculosis is a non-canonical heme-degrading enzyme to acquire iron from heme

Some Gram-negative pathogens import host heme into the cytoplasm and utilize it as an iron source for their survival. We report here that HmuS, encoded by the heme utilizing system (hmu) locus, cleaves the protoporphyrin ring to release iron from heme. A liquid chromatography/mass spectrometry analysis revealed that the degradation products of this reaction are two biliverdin isomers that result from transformation of a verdoheme intermediate. This oxidative heme degradation by HmuS required molecular oxygen and electrons supplied by either ascorbate or NADPH. Electrons could not be directly transferred from NADPH to heme; instead, ferredoxin-NADP⁺ reductase (FNR) functioned as a mediator. Although HmuS does not share amino acid sequence homology with heme oxygenase (HO), a well-known heme-degrading enzyme, absorption and resonance Raman spectral analyses suggest that the heme iron is coordinated with an axial histidine residue and a water molecule in both enzymes. The substitution of axial His196 or distal Arg102 with an alanine residue in HmuS almost completely eliminated heme-degradation activity, suggesting that Fe-His coordination and interaction of a distal residue with water molecules in the heme pocket are important for this activity.

Structure and dynamics of Type III periplasmic proteins VcFhuD and VcHutB reveal molecular basis of their distinctive ligand binding properties

Molecular mechanisms of xenosiderophore and heme acquisitions using periplasmic binding protein (PBP) dependent ATP-binding cassette transporters to scavenge the essential nutrient iron are elusive yet in Vibrio cholerae. Our current study delineates the structures, dynamics and ligand binding properties of two Type III PBPs of V. cholerae, VcFhuD and VcHutB. Through crystal structures and fluorescence quenching studies we demonstrate unique features of VcFhuD to bind both hydroxamate and catecholate type xenosiderophores. Like E. coli FhuD, VcFhuD binds ferrichrome and ferri-desferal using conserved Tryptophans and R102. However, unlike EcFhuD, slightly basic ligand binding pocket of VcFhuD could favour ferri-enterobactin binding with plausible participation of R203, along with R102, like it happens in catecholate binding PBPs. Structural studies coupled with spectrophotometric and native PAGE analysis indicated parallel binding of two heme molecules to VcHutB in a pH dependent manner, while mutational analysis established the relative importance of Y65 and H164 in heme binding. MD simulation studies exhibited an unforeseen inter-lobe swinging motion in Type III PBPs, magnitude of which is inversely related to the packing of the linker helix with its neighboring helices. Small inter-lobe movement in VcFhuD or dramatic twisting in VcHutB is found to influence ligand binding.

A Novel Role for Progesterone Receptor Membrane Component 1 (PGRMC1): A Partner and Regulator of Ferrochelatase

Heme is an iron-containing cofactor essential for multiple cellular processes and fundamental activities such as oxygen transport. To better understand the means by which heme synthesis is regulated during erythropoiesis, affinity purification coupled with mass spectrometry (MS) was performed to identify putative protein partners interacting with ferrochelatase (FECH), the terminal enzyme in the heme biosynthetic pathway. Both progesterone receptor membrane component 1 (PGRMC1) and progesterone receptor membrane component 2 (PGRMC2) were identified in these experiments. These interactions were validated by reciprocal affinity purification followed by MS analysis and immunoblotting. The interaction between PGRMC1 and FECH was confirmed in vitro and in HEK 293T cells, a non-erythroid cell line. When cells that are recognized models for erythroid differentiation were treated with a small molecule inhibitor of PGRMC1, AG-205, there was an observed decrease in the level of hemoglobinization relative to that of untreated cells. In vitro heme transfer experiments showed that purified PGRMC1 was able to donate heme to apo-cytochrome b5. In the presence of PGRMC1, in vitro measured FECH activity decreased in a dose-dependent manner. Interactions between FECH and PGRMC1 were strongest for the conformation of FECH associated with product release, suggesting that PGRMC1 may regulate FECH activity by controlling heme release. Overall, the data illustrate a role for PGRMC1 in regulating heme synthesis via interactions with FECH and suggest that PGRMC1 may be a heme chaperone or sensor.

Purification, crystallization and preliminary X-ray analysis of the periplasmic haem-binding protein HutB from Vibrio cholerae

The mechanism of haem transport across the inner membrane of pathogenic bacteria is currently insufficiently understood at the molecular level and no information is available for this process in Vibrio cholerae. To obtain structural insights into the periplasmic haem-binding protein HutB from V. cholerae (VcHutB), which is involved in haem transport through the HutBCD haem-transport system, at the atomic level, VcHutB was cloned, overexpressed and crystallized using 1.6 M ammonium sulfate as a precipitant at pH 7.0. X-ray diffraction data were collected to 2.4 Å resolution on the RRCAT PX-BL-21 beamline at the Indus-2 synchrotron, Indore, India. The crystals belonged to space group P4₃2₁2, with unit-cell parameters a = b = 62.88, c = 135.8 Å. Matthews coefficient calculations indicated the presence of one monomer in the asymmetric unit, with an approximate solvent content of 45.02%. Molecular-replacement calculations with Phaser confirmed the presence of a monomer in the asymmetric unit.

Spectroscopic and mutagenesis studies of human PGRMC1

Progesterone receptor membrane component 1 (PGRMC1) is a 25 kDa protein with an N-terminal transmembrane domain and a putative C-terminal cytochrome b5 domain. Heme-binding activity of PGRMC1 has been shown in various homologues of PGRMC1. Although the general definition of PGRMC1 is as a progesterone receptor, progesterone-binding activity has not been directly demonstrated in any of the purified PGRMC1 proteins fully loaded with heme. Here, we show that the human homologue of PGRMC1 (hPGRMC1) binds heme in a five-coordinate (5C) high-spin (HS) configuration, with an axial tyrosinate ligand, likely Y95. The negatively charged tyrosinate ligand leads to a relatively low redox potential of approximately -331 mV. The Y95C or Y95F mutation dramatically reduces the ability of the protein to bind heme, supporting the assignment of the axial heme ligand to Y95. On the other hand, the Y95H mutation retains ∼90% of the heme-binding activity. The heme in Y95H is also 5CHS, but it has a hydroxide axial ligand, conceivably stabilized by the engineered-in H95 via an H-bond; CO binding to the distal ligand-binding site leads to an exchange of the axial ligand to a histidine, possibly H95. We show that progesterone binds to hPGRMC1 and introduces spectral changes that manifest conformational changes to the heme. Our data offer the first direct evidence supporting progesterone-binding activity of PGRMC1.

Shared and distinct mechanisms of iron acquisition by bacterial and fungal pathogens of humans

Iron is the most abundant transition metal in the human body and its bioavailability is stringently controlled. In particular, iron is tightly bound to host proteins such as transferrin to maintain homeostasis, to limit potential damage caused by iron toxicity under physiological conditions and to restrict access by pathogens. Therefore, iron acquisition during infection of a human host is a challenge that must be surmounted by every successful pathogenic microorganism. Iron is essential for bacterial and fungal physiological processes such as DNA replication, transcription, metabolism, and energy generation via respiration. Hence, pathogenic bacteria and fungi have developed sophisticated strategies to gain access to iron from host sources. Indeed, siderophore production and transport, iron acquisition from heme and host iron-containing proteins such as hemoglobin and transferrin, and reduction of ferric to ferrous iron with subsequent transport are all strategies found in bacterial and fungal pathogens of humans. This review focuses on a comparison of these strategies between bacterial and fungal pathogens in the context of virulence and the iron limitation that occurs in the human body as a mechanism of innate nutritional defense.

Role and regulation of heme iron acquisition in gram-negative pathogens

Bacteria that reside in animal tissues and/or cells must acquire iron from their host. However, almost all of the host iron is sequestered in iron-containing compounds and proteins, the majority of which is found within heme molecules. Thus, likely iron sources for bacterial pathogens (and non-pathogenic symbionts) are free heme and heme-containing proteins. Furthermore, the cellular location of the bacterial within the host (intra or extracellular) influences the amount and nature of the iron containing compounds available for transport. The low level of free iron in the host, coupled with the presence of numerous different heme sources, has resulted in a wide range of high-affinity iron acquisition strategies within bacteria. However, since excess iron and heme are toxic to bacteria, expression of these acquisition systems is highly regulated. Precise expression in the correct host environment at the appropriate times enables heme iron acquisitions systems to contribute to the growth of bacterial pathogens within the host. This mini-review will highlight some of the recent findings in these areas for gram-negative pathogens.

Heme uptake and metabolism in bacteria

All but a few bacterial species have an absolute need for heme, and most are able to synthesize it via a pathway that is highly conserved among all life domains. Because heme is a rich source for iron, many pathogenic bacteria have also evolved processes for sequestering heme from their hosts. The heme biosynthesis pathways are well understood at the genetic and structural biology levels. In comparison, much less is known about the heme acquisition, trafficking, and degradation processes in bacteria. Gram-positive and Gram-negative bacteria have evolved similar strategies but different tactics for importing and degrading heme, likely as a consequence of their different cellular architectures. The differences are manifested in distinct structures for molecules that perform similar functions. Consequently, the aim of this chapter is to provide an overview of the structural biology of proteins and protein-protein interactions that enable Gram-positive and Gram-negative bacteria to sequester heme from the extracellular milieu, import it to the cytosol, and degrade it to mine iron.

Why orange Guaymas Basin Beggiatoa spp. are orange: single-filament-genome-enabled identification of an abundant octaheme cytochrome with hydroxylamine oxidase, hydrazine oxidase, and nitrite reductase activities

Orange, white, and yellow vacuolated Beggiatoaceae filaments are visually dominant members of microbial mats found near sea floor hydrothermal vents and cold seeps, with orange filaments typically concentrated toward the mat centers. No marine vacuolate Beggiatoaceae are yet in pure culture, but evidence to date suggests they are nitrate-reducing, sulfide-oxidizing bacteria. The nearly complete genome sequence of a single orange Beggiatoa ("Candidatus Maribeggiatoa") filament from a microbial mat sample collected in 2008 at a hydrothermal site in Guaymas Basin (Gulf of California, Mexico) was recently obtained. From this sequence, the gene encoding an abundant soluble orange-pigmented protein in Guaymas Basin mat samples (collected in 2009) was identified by microcapillary reverse-phase high-performance liquid chromatography (HPLC) nano-electrospray tandem mass spectrometry (μLC-MS-MS) of a pigmented band excised from a denaturing polyacrylamide gel. The predicted protein sequence is related to a large group of octaheme cytochromes whose few characterized representatives are hydroxylamine or hydrazine oxidases. The protein was partially purified and shown by in vitro assays to have hydroxylamine oxidase, hydrazine oxidase, and nitrite reductase activities. From what is known of Beggiatoaceae physiology, nitrite reduction is the most likely in vivo role of the octaheme protein, but future experiments are required to confirm this tentative conclusion. Thus, while present-day genomic and proteomic techniques have allowed precise identification of an abundant mat protein, and its potential activities could be assayed, proof of its physiological role remains elusive in the absence of a pure culture that can be genetically manipulated.

Heme binding properties of glyceraldehyde-3-phosphate dehydrogenase

Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) is a glycolytic enzyme that also functions in transcriptional regulation, oxidative stress, vesicular trafficking, and apoptosis. Because GAPDH is required for the insertion of cellular heme into inducible nitric oxide synthase [Chakravarti, R., et al. (2010) Proc. Natl. Acad. Sci. U.S.A. 107, 18004-18009], we extensively characterized the heme binding properties of GAPDH. Substoichiometric amounts of ferric heme bound to GAPDH (one heme per GAPDH tetramer) to form a low-spin complex with UV-visible maxima at 362, 418, and 537 nm and when reduced to ferrous gave maxima at 424, 527, and 559 nm. Ferric heme association and dissociation rate constants at 10 °C were as follows: k(on) = 17800 M(-1) s(-1), k(off1) = 7.0 × 10(-3) s(-1), and k(off2) = 3.3 × 10(-4) s(-1) (giving approximate affinities of 19-390 nM). Ferrous heme bound more poorly to GAPDH and dissociated with a k(off) of 4.2 × 10(-3) s(-1). Magnetic circular dichroism, resonance Raman, and electron paramagnetic resonance spectroscopic data on the ferric, ferrous, and ferrous-CO complexes of GAPDH showed that the heme is bis-ligated with His as the proximal ligand. The distal ligand in the ferric complex was not displaced by CN(-) or N(3)(-) but in the ferrous complex could be displaced by CO at a rate of 1.75 s(-1) (for >0.2 mM CO). Studies with heme analogues revealed selectivity toward the coordinating metal and porphyrin ring structure. The GAPDH-heme complex was isolated from bacteria induced to express rabbit GAPDH in the presence of δ-aminolevulinic acid. Our finding of heme binding to GAPDH expands the protein's potential roles. The strength, selectivity, reversibility, and redox sensitivity of heme binding to GAPDH are consistent with it performing heme sensing or heme chaperone-like functions in cells.

Sortase independent and dependent systems for acquisition of haem and haemoglobin in Listeria monocytogenes

We studied three Fur-regulated systems of Listeria monocytogenes: the srtB region, that encodes sortase-anchored proteins and a putative ABC transporter, and the fhu and hup operons, that produce putative ABC transporters for ferric hydroxamates and haemin (Hn)/haemoglobin (Hb) respectively. Deletion of lmo2185 in the srtB region reduced listerial [(59) Fe]-Hn transport, and purified Lmo2185 bound [(59) Fe]-Hn (K(D) = 12 nM), leading to its designation as a Hn/Hb binding protein (hbp2). Purified Hbp2 also acted as a haemophore, capturing and supplying Hn from the environment. Nevertheless, Hbp2 only functioned in [(59) Fe]-Hn transport at external concentrations less than 50 nM: at higher Hn levels its uptake occurred with equivalent affinity and rate without Hbp2. Similarly, deletion of sortase A had no effect on ferric siderophore or Hn/Hb transport at any concentration, and the srtA-independence of listerial Hn/Hb uptake distinguished it from comparable systems of Staphylococcus aureus. In the cytoplasmic membrane, the Hup transporter was specific for Hn: its lipoprotein (HupD) only showed high affinity for the iron porphyrin (K(D) = 26 nM). Conversely, the FhuD lipoprotein encoded by the fhu operon had broad specificity: it bound both ferric siderophores and Hn, with the highest affinity for ferrioxamine B (K(D) = 123 nM). Deletions of Hup permease components hupD, hupG or hupDGC reduced Hn/Hb uptake, and complementation of ΔhupC and ΔhupG by chromosomal integration of hupC(+) and hupG(+) alleles on pPL2 restored growth promotion by Hn/Hb. However, ΔhupDGC did not completely eliminate [(59) Fe]-Hn transport, implying the existence of another cytoplasmic membrane Hn transporter. The overall K(M) of Hn uptake by wild-type strain EGD-e was 1 nM, and it occurred at similar rates (V(max) = 23 pmol 10(9) cells(-1) min(-1)) to those of ferric siderophore transporters. In the ΔhupDGC strain uptake occurred at a threefold lower rate (V(max) = 7 pmol 10(9) cells(-1) min(-1)). The results show that at low (< 50 nM) levels of Hn, SrtB-dependent peptidoglycan-anchored proteins (e.g. Hbp2) bind the porphyrin, and HupDGC or another transporter completes its uptake into the cytoplasm. However, at higher concentrations Hn uptake is SrtB-independent: peptidoglycan-anchored binding proteins are dispensable because HupDGC directly absorbs and internalizes Hn. Finally, ΔhupDGC increased the LD(50) of L. monocytogenes 100-fold in the mouse infection model, reiterating the importance of this system in listerial virulence.

Recent advances in bacterial heme protein biochemistry

Recent progress in genetics, fed by the burst in genome sequence data, has led to the identification of a host of novel bacterial heme proteins that are now being characterized in structural and mechanistic terms. The following short review highlights very recent work with bacterial heme proteins involved in the uptake, biosynthesis, degradation, and use of heme in respiration and sensing.

Mechanisms of iron import in anthrax

During an infection, bacterial pathogens must acquire iron from the host to survive. However, free iron is sequestered in host proteins, which presents a barrier to iron-dependent bacterial replication. In response, pathogens have developed mechanisms to acquire iron from the host during infection. Interestingly, a significant portion of the iron pool is sequestered within heme, which is further bound to host proteins such as hemoglobin. The copious amount of heme-iron makes hemoglobin an ideal molecule for targeted iron uptake during infection. While the study of heme acquisition is well represented in Gram-negative bacteria, the systems and mechanism of heme uptake in Gram-positive bacteria has only recently been investigated. Bacillus anthracis, the causative agent of anthrax disease, represents an excellent model organism to study iron acquisition processes owing to a multifaceted lifecycle consisting of intra- and extracellular phases and a tremendous replicative potential upon infection. This review provides an in depth description of the current knowledge of B. anthracis iron acquisition and applies these findings to a general understanding of how pathogenic Gram-positive bacteria transport this critical nutrient during infection.

Overcoming the heme paradox: heme toxicity and tolerance in bacterial pathogens

Virtually all bacterial pathogens require iron to infect vertebrates. The most abundant source of iron within vertebrates is in the form of heme as a cofactor of hemoproteins. Many bacterial pathogens have elegant systems dedicated to the acquisition of heme from host hemoproteins. Once internalized, heme is either degraded to release free iron or used intact as a cofactor in catalases, cytochromes, and other bacterial hemoproteins. Paradoxically, the high redox potential of heme makes it a liability, as heme is toxic at high concentrations. Although a variety of mechanisms have been proposed to explain heme toxicity, the mechanisms by which heme kills bacteria are not well understood. Nonetheless, bacteria employ various strategies to protect against and eliminate heme toxicity. Factors involved in heme acquisition and detoxification have been found to contribute to virulence, underscoring the physiological relevance of heme stress during pathogenesis. Herein we describe the current understanding of the mechanisms of heme toxicity and how bacterial pathogens overcome the heme paradox during infection.

The 2-Cys peroxiredoxin alkyl hydroperoxide reductase c binds heme and participates in its intracellular availability in Streptococcus agalactiae

Heme is a redox-reactive molecule with vital and complex roles in bacterial metabolism, survival, and virulence. However, few intracellular heme partners were identified to date and are not well conserved in bacteria. The opportunistic pathogen Streptococcus agalactiae (group B Streptococcus) is a heme auxotroph, which acquires exogenous heme to activate an aerobic respiratory chain. We identified the alkyl hydroperoxide reductase AhpC, a member of the highly conserved thiol-dependent 2-Cys peroxiredoxins, as a heme-binding protein. AhpC binds hemin with a K(d) of 0.5 microm and a 1:1 stoichiometry. Mutagenesis of cysteines revealed that hemin binding is dissociable from catalytic activity and multimerization. AhpC reductase activity was unchanged upon interaction with heme in vitro and in vivo. A group B Streptococcus ahpC mutant displayed attenuation of two heme-dependent functions, respiration and activity of a heterologous catalase, suggesting a role for AhpC in heme intracellular fate. In support of this hypothesis, AhpC-bound hemin was protected from chemical degradation in vitro. Our results reveal for the first time a role for AhpC as a heme-binding protein.

Bacterial heme-transport proteins and their heme-coordination modes

Efficient iron acquisition is critical for an invading microbe's survival and virulence. Most of the iron in mammals is incorporated into heme, which can be plundered by certain bacterial pathogens as a nutritional iron source. Utilization of exogenous heme by bacteria involves the binding of heme or hemoproteins to the cell surface receptors, followed by the transport of heme into cells. Once taken into the cytosol, heme is presented to heme oxygenases where the tetrapyrrole ring is cleaved in order to release the iron. Some Gram-negative bacteria also secrete extracellular heme-binding proteins called hemophores, which function to sequester heme from the environment. The heme-transport genes are often genetically linked as gene clusters under Fur (ferric uptake regulator) regulation. This review discusses the gene clusters and proteins involved in bacterial heme acquisition, transport and processing processes, with special focus on the heme-coordination, protein structures and mechanisms underlying heme-transport.

Functional differences between heme permeases: Serratia marcescens HemTUV permease exhibits a narrower substrate specificity (restricted to heme) than the Escherichia coli DppABCDF peptide-heme permease

Serratia marcescens hemTUV genes encoding a potential heme permease were cloned in Escherichia coli recombinant mutant FB827 dppF::Km(pAM 238-hasR). This strain, which expresses HasR, a foreign heme outer membrane receptor, is potentially capable of using heme as an iron source. However, this process is invalidated due to a dppF::Km mutation which inactivates the Dpp heme/peptide permease responsible for heme, dipeptide, and delta-aminolevulinic (ALA) transport through the E. coli inner membrane. We show here that hemTUV genes complement the Dpp permease for heme utilization as an iron source and thus are functional in E. coli. However, hemTUV genes do not complement the Dpp permease for ALA uptake, indicating that the HemTUV permease does not transport ALA. Peptides do not inhibit heme uptake in vivo, indicating that, unlike Dpp permease, HemTUV permease does not transport peptides. HemT, the periplasmic binding protein, binds heme. Heme binding is saturable and not inhibited by peptides that inhibit heme uptake by the Dpp system. Thus, the S. marcescens HemTUV permease and, most likely, HemTUV orthologs present in many gram-negative pathogens form a class of heme-specific permeases different from the Dpp peptide/heme permease characterized in E. coli.

Measurement of the heme affinity for yeast dap1p, and its importance in cellular function

Current studies on the Saccharomyces cerevisiae protein Dap1p have demonstrated a heme-related function within the ergosterol biosynthetic pathway. Here we present data to further the understanding of the role of heme in the proper biological functioning of Dap1p in cellular processes. First, we examined the role of Dap1p in stabilizing the P450 enzyme, Erg11p, a key regulatory protein in ergosterol biosynthesis. Our data indicate that the absence of Dap1p does not affect Erg11p mRNA, protein expression levels, or the protein degradation rates in S. Cerevisaie. Second, in order to probe the role of heme in the biological functioning of Dap1p, we measured ferric and ferrous heme binding affinities for Dap1p and the mutant Dap1pY138F, as well as equilibrium midpoint reduction potentials of the Fe(III)/Fe(II) couples. Our results show that both wild-type and mutant proteins bind heme in a 1:1 fashion, possessing tight ferric heme affinities, KD values of 400 pM and 200 nM, respectively, but exhibiting weak ferrous affinities, 2 and 10 microM, respectively. Additionally, the measured reduction potential of Dap1p, which was found to be -307 mV, is similar to that of other monotyrosinate hemoproteins. Although previous reports show the weaker affinity of Dap1pY138F for ferric heme lowers the production of ergosterol with respect to wild-type Dap1p in S. pombe, we find that Dap1pY138F expression is still sufficient to rescue the growth sensitivity of dap1Delta to fluconazole and methyl methanesulfonate in S. cerevisiae. Various interpretations of these results are discussed with respect to Dap1p function in the cell.